The blades off of an 18 will be a direct swap if the holes are drilled for the new-style castings. If they're drilled for the old-style castings, then you'd have to fill the existing holes with epoxy and re-drill in the correct hole locations. But regardless, the blades themselves are interchangeable.

The rudder blades for the Tiger are a different profile, but they use the same rudder casting as the 17, so they should fit. However they wouldn't be class legal. They'd probably be fine for "fun" sailing though.